primes =[[2], [3], [5], [7], [11], [13], [17], [19], [23], [29], [31], [37], [41], [43], [47], [51], [59], [61], [67], [71], [73], [79], [83], [89], [97], [101], [103], [107], [109], [113], [127], [131], [137], [139], [149], [151], [157], [163], [167], [173], [179], [181], [191], [193], [197], [199], [211], [223], [227], [229], [233], [239], [241], [251], [257], [263], [269], [271], [277], [281], [283], [293], [307], [311], [313], [317], [331], [337], [347], [349], [353], [359], [367], [373], [379], [383], [389], [397], [401], [409], [419], [421], [431], [433], [439], [443], [449], [457], [461], [463], [467], [479], [487], [491], [499], [503], [509], [521], [523], [541], [547], [557], [563], [569], [571], [577], [587], [593], [599], [601], [607], [613], [617], [619], [631], [641], [643], [647], [653], [659], [661], [673], [677], [683], [691], [701], [709], [719], [727], [733], [739], [743], [751], [757], [761], [769], [773], [787], [797], [809], [811], [821], [823], [827], [829], [839], [853], [857], [859], [863], [877], [881], [883], [887], [907], [911], [919], [929], [937], [941], [947], [953], [967], [971], [977], [983], [991], [997], [1009], [1013], [1019], [1021], [1031], [1033], [1039], [1049], [1051], [1061], [1063], [1069], [1087], [1091], [1093], [1097], [1103], [1109], [1117], [1123], [1129], [1151], [1153], [1163], [1171], [1181], [1187], [1193], [1201], [1213], [1217], [1223], [1229], [1231], [1237], [1249], [1259], [1277], [1279], [1283], [1289], [1291], [1297], [1301], [1303], [1307], [1319], [1321], [1327], [1361], [1367], [1373], [1381], [1399], [1409], [1423], [1427], [1429], [1433], [1437], [1447], [1451], [1453], [1459], [1471], [1481], [1483], [1487], [1489], [1493], [1499], [1511], [1523], [1531], [1543], [1549], [1553], [1559], [1567], [1571], [1579], [1583], [1597], [1601], [1607], [1609], [1613], [1619], [1621], [1627], [1637], [1657], [1663], [1667], [1669], [1693], [1697], [1699], [1709], [1721], [1723], [1733], [1741], [1747], [1753], [1759], [1777], [1783], [1787], [1789], [1801], [1811], [1823], [1831], [1847], [1861], [1867], [1871], [1873], [1877], [1879], [1889], [1901], [1907], [1913], [1931], [1933], [1949], [1951], [1973], [1979], [1987], [1993], [1997], [1999], [2003], [2011], [2017], [2027], [2029], [2039], [2053], [2063], [2069], [2081], [2083], [2087], [2089], [2099], [2111], [2113], [2129], [2131], [2137], [2141], [2143], [2153], [2161], [2179], [2203], [2207], [2213], [2221], [2237], [2239], [2243], [2251], [2267], [2269], [2273], [2281], [2287], [2293], [2297], [2309], [2311], [2333], [2339], [2341], [2347], [2351], [2357], [2371], [2377], [2381], [2383], [2389], [2393], [2399], [2411], [2417], [2423], [2437], [2441], [2447], [2459], [2467], [2473], [2477], [2503], [2521], [2531], [2539], [2543], [2549], [2551], [2557], [2579], [2591], [2593], [2609], [2617], [2621], [2633], [2647], [2657], [2659], [2663], [2671], [2677], [2683], [2687], [2689], [2693], [2699], [2707], [2711], [2713], [2719], [2729], [2731], [2741], [2749], [2753], [2767], [2777], [2789], [2791], [2797], [2801], [2803], [2819], [2833], [2837], [2843], [2851], [2857], [2861], [2879], [2887], [2897], [2903], [2909], [2917], [2927], [2939], [2953], [2957], [2963], [2969], [2971], [2999], [3001], [3011], [3019], [3023], [3037], [3041], [3049], [3061], [3067], [3079], [3083], [3089], [3109], [3119], [3121], [3137], [3163], [3167], [3169], [3181], [3187], [3191], [3203], [3209], [3217], [3221], [3229], [3251], [3253], [3257], [3259], [3271], [3299], [3301], [3307], [3313], [3319], [3323], [3329], [3331], [3343], [3347], [3359], [3361], [3371], [3373], [3389], [3391], [3407], [3413], [3433], [3449], [3457], [3461], [3463], [3467], [3469], [3491], [3499], [3511], [3517], [3527], [3529], [3533], [3539], [3541], [3547], [3557], [3559], [3571], [3581], [3583], [3593], [3607], [3613], [3617], [3623], [3631], [3637], [3643], [3659], [3671], [3673], [3677], [3691], [3697], [3701], [3709], [3719], [3727], [3733], [3739], [3761], [3767], [3769], [3779], [3793], [3797], [3803], [3821], [3823], [3833], [3847], [3851], [3853],[3863], [3877], [3881], [3889], [3907], [3911], [3917], [3919], [3923], [3929], [3931], [3943], [3947], [3967], [3989], [4001], [4003], [4007], [4013], [4019], [4021], [4027], [4049], [4051], [4057], [4073], [4079], [4091], [4093], [4099], [4111], [4127], [4129], [4133], [4139], [4153], [4157], [4159], [4177], [4201], [4211], [4217], [4219], [4229], [4231], [4241], [4243], [4253], [4259], [4261], [4271], [4273], [4283], [4289], [4297], [4327], [4337], [4339], [4349], [4357], [4363], [4373], [4391], [4397], [4409], [4421], [4423], [4441], [4447], [4451], [4457], [4463], [4481], [4483], [4493], [4507], [4513], [4517], [4519], [4523], [4547], [4549], [4561], [4567], [4583], [4591], [4597], [4603], [4621], [4637], [4639], [4643], [4649], [4651], [4657], [4663], [4673], [4679], [4691], [4703], [4721], [4723], [4729], [4733], [4751], [4759], [4783], [4787], [4789], [4793], [4799], [4801], [4813], [4817], [4831], [4861], [4871], [4877], [4889], [4903], [4909], [4919], [4931], [4933], [4937], [4943], [4951], [4957], [4967], [4969], [4973], [4987], [4993], [4999], [5003], [5009], [5011], [5021], [5023], [5039], [5051], [5059], [5077], [5081], [5087], [5099], [5101], [5107], [5113], [5119], [5147], [5153], [5167], [5171], [5179], [5189], [5197], [5209], [5227], [5231], [5233], [5237], [5261], [5273], [5279], [5281], [5297], [5303], [5309], [5323], [5333], [5347], [5351], [5381], [5387], [5393], [5399], [5407], [5413], [5417], [5419], [5431], [5437], [5441], [5443], [5449], [5471], [5477], [5479], [5483], [5501], [5503], [5507], [5519], [5521], [5527], [5531], [5557], [5563], [5569], [5573], [5581], [5591], [5623], [5639], [5641], [5647], [5651], [5653], [5657], [5659], [5669], [5683], [5689], [5693], [5701], [5711], [5717], [5737], [5741], [5743], [5749], [5779], [5783], [5791], [5801], [5807], [5813], [5821], [5827], [5839], [5843], [5849], [5851], [5857], [5861], [5867], [5869], [5879], [5881], [5897], [5903], [5923], [5927], [5939], [5953], [5981], [5987], [6007], [6011], [6029], [6037], [6043], [6047], [6053], [6067], [6073], [6079], [6089], [6091], [6101], [6113], [6121], [6131], [6133], [6143], [6151], [6163], [6173], [6197], [6199], [6203], [6211], [6217], [6221], [6229], [6247], [6257], [6263], [6269], [6271], [6277], [6287], [6299], [6301], [6311], [6317], [6323], [6329], [6337], [6343], [6353], [6359], [6361], [6367], [6373], [6379], [6389], [6397], [6421], [6427], [6449], [6451], [6469], [6473], [6481], [6491], [6521], [6529], [6547], [6551], [6553], [6563], [6569], [6571], [6577], [6581], [6599], [6607], [6619], [6637], [6653], [6659], [6661], [6673], [6679], [6689], [6691], [6701], [6703], [6709], [6719], [6733], [6737], [6761], [6763], [6779], [6781], [6791], [6793], [6803], [6823], [6827], [6829], [6833], [6841], [6857], [6863], [6869], [6871], [6883], [6899], [6907], [6911], [6917], [6947], [6949], [6959], [6961], [6967], [6971], [6977], [6983], [6991], [6997], [7001], [7013], [7019], [7027], [7039], [7043], [7057], [7069], [7079], [7103], [7109], [7121], [7127], [7129], [7151], [7159], [7177], [7187], [7193], [7207], [7211], [7213], [7219], [7229], [7237], [7243], [7247], [7253], [7283], [7297], [7307], [7309], [7321], [7331], [7333], [7349], [7351], [7369], [7393], [7411], [7417], [7433], [7451], [7457], [7459], [7477], [7481], [7487], [7489], [7499], [7507], [7517], [7523], [7529], [7537], [7541], [7547], [7549], [7559], [7561], [7573], [7577], [7583], [7589], [7591], [7603], [7607], [7621], [7639], [7643], [7649], [7669], [7673], [7681], [7687], [7691], [7699], [7703], [7717], [7723], [7727], [7741], [7753], [7757], [7759], [7789], [7793], [7817], [7823], [7829], [7841], [7853], [7867], [7873], [7877], [7879], [7883], [7901], [7907], [7919], [7927], [7933], [7937], [7949], [7951], [7963], [7993], [8009], [8011], [8017], [8039], [8053], [8059], [8069], [8081], [8087], [8089], [8093], [8101], [8111], [8117], [8123], [8147], [8161], [8167], [8171], [8179], [8191], [8209], [8219], [8221], [8231], [8233], [8237], [8243], [8263], [8269], [8273], [8287], [8291], [8293], [8297], [8311], [8317], [8329], [8353], [8363],[8369], [8377], [8387], [8389], [8419], [8423], [8429], [8431], [8443], [8447], [8461], [8467], [8501], [8513], [8521], [8527], [8537], [8539], [8543], [8563], [8573], [8581], [8597], [8599], [8609], [8623], [8627], [8629], [8641], [8647], [8663], [8669], [8677], [8681], [8689], [8693], [8699], [8707], [8713], [8719], [8731], [8737], [8741], [8747], [8753], [8761], [8779], [8783], [8803], [8807], [8819], [8821], [8831], [8837], [8839], [8849], [8861], [8863], [8867], [8887], [8893], [8923], [8929], [8933], [8941], [8951], [8963], [8969], [8971], [8999], [9001], [9007], [9011], [9013], [9029], [9041], [9043], [9049], [9059], [9067], [9091], [9103], [9109], [9127], [9133], [9137], [9151], [9157], [9161], [9173], [9181], [9187], [9199], [9203], [9209], [9221], [9227], [9239], [9241], [9257], [9277], [9281], [9283], [9293], [9311], [9319], [9323], [9337], [9341], [9343], [9349], [9371], [9377], [9391], [9397], [9403], [9413], [9419], [9421], [9431], [9433], [9437], [9439], [9461], [9463], [9467], [9473], [9479], [9491], [9497], [9511], [9521], [9533], [9539], [9547], [9551], [9587], [9601], [9613], [9619], [9623], [9629], [9631], [9643], [9649], [9661], [9677], [9679], [9689], [9697], [9719], [9721], [9733], [9739], [9743], [9749], [9767], [9769], [9781], [9787], [9791], [9803], [9811], [9817], [9829], [9833], [9839], [9851], [9857], [9859], [9871], [9883], [9887], [9901], [9907], [9923], [9929], [9931], [9941], [9949], [9967], [9973]]
num = input()
maxim = 0
if num[0] in primes and int(num[0]) > maxim :
   maxim = num[0]
    
elif num[1] in primes and int(num[1]) > maxim :
   maxim = num[1]
    
elif num[2] in primes and int(num[2]) > maxim :
    maxim = num[2]

elif num[3] in primes and int(num[3]) > maxim :
    maxim = num[3]
    
elif num[0]+num[1] in primes and int(num[0]+num[1]) > maxim:
    maxim = num[0]+num[1]

elif num[1]+num[2] in primes and int(num[1]+num[2]) > maxim:
    maxim = num[1]+num[2]
    
elif num[2]+num[3] in primes and int(num[2]+num[3]) > maxim:
    maxim = num[2]+num[3]
   
elif num[0]+num[1]+num[2] in primes and int(num[0]+num[1]+num[2]) > maxim:
    maxim = num[0]+num[1]+num[2]
    
elif num[1]+num[2]+num[3] in primes and int(num[1]+num[2]+num[3]) > maxim:
    maxim = num[1]+num[2]+num[3]
    
elif num in primes and int(num) > maxim:
    maxim = num
    

print(maxim)